1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a radio frequency identification (RFID) reader, and more particularly, to an RFID reader which addresses problems of signal deterioration and DC offset by effectively removing a transmission carrier leakage signal being inputted from a transmitting circuit to a receiving circuit, and which has a simple circuit construction.
2. Description of the Related Art
A ubiquitous sensor network (USN) has recently received a lot of attention as the basic infrastructure for realizing a ubiquitous society, which is one of the big issues in the information and communication field. The USN is a technology that adopts an electronic tag having a communication function, which is attached to every object, that detects neighboring environmental information on the basis of recognized information of the object obtained through the electronic tag, and manages and uses the detected information in real time through a network.
The core of the USN is an RFID system, which is composed of a reader, an antenna, an electronic tag, a server, and a network. Here, the reader serves to read information stored in the electronic tag or store information in the electronic tag, and the antenna exchanges data stored in the electronic tag by defined frequencies and protocols.
FIG. 1 is a schematic circuit diagram of a related RFID system composed of an RFID tag and an RFID reader 10. As illustrated in FIG. 1, the RFID reader 10 includes a transmitting circuit 20, a receiving circuit 25, a phase locked loop (PLL) 21, and a processing circuit 27.
The transmitting circuit 20 generates a transmitted signal of a specified frequency that is transferred to the RFID tag 30, and the receiving circuit 25 receives a received signal reflected from the RFID tag 30.
The PLL 21 is a circuit that detects a phase difference between the transmitted signal and the received signal, and controls the phase of a synthesizer using a voltage, which is in proportion to the phase difference, so that the phase of the received signal and the phase of the transmitted signal become equal to each other.
The processing circuit 27 controls the transmitting circuit 20 to transmit the transmitted signal to the RFID tag 30, and acquires information of the RFID tag 30 by reading the signal received in the receiving signal 25.
The transmitting circuit 20 and the receiving circuit 25 includes an antenna 11, a filter 13, and a DC coupler 15. The antenna 11 transmits the transmitted signal from the transmitted circuit 20 to the RFID tag 30, and receives the received signal reflected from the RFID tag 30 to transfer the received signal to the receiving circuit 25. The filter 13 filters the transmitted signal and the received signal with a desired size, and the DC coupler 15 adjusts the DC voltages of the transmitted signal and the received signal.
Generally, in the case of a wireless appliance, the transmitted circuit 20 and the receiving circuit 25 operate separately, and the signal transmitted form the transmitting circuit 20 and the signal received in the receiving circuit 25 have frequencies different from each other. Accordingly, the receiving circuit 25 can be in an off state while the transmitting circuit 20 operates, and this prevents the signal transmitted from the transmitting circuit 20 from being directly inputted to the receiving circuit 25.
By contrast, in the case of the RFID system, since the transmitting circuit 20 and the receiving circuit 25 use the same frequency and the distance to the RFID tag 30 cannot be known, the transmitting circuit 20 and the receiving circuit 25 are simultaneously in an on state. If the transmitted signal as illustrated in (a) of FIG. 2 is transmitted from the transmitting circuit 20 in a state that both the transmitting circuit 20 and the receiving circuit 25 are simultaneously in an on state, a transmission carrier leakage signal as illustrated in (d) of FIG. 2 is directly inputted to the receiving circuit 25 through the antenna.
If the transmission carrier leakage signal is inputted to the receiving circuit 25 as described above, it acts as a noise, and a signal obtained by addition of the transmission carrier leakage signal to the received signal from the RFID tag 30 as illustrated in (b) of FIG. 2 is received in the receiving circuit 25, as illustrated in (e) of FIG. 2. Accordingly, although the received signal that can be obtained by filtering when no transmission carrier leakage signal exists is as shown in (c) of FIG. 2, the received signal that can be obtained by filtering when the transmission carrier leakage signal exist is as shown in (f) of FIG. 2, so that it is almost impossible to obtain the received signal from the RFID tag 30. As a result, problems of signal deterioration and DC offset may occur.
In addition, each component of the receiving circuit 25, e.g., a low noise amplifier (LNA) or a mixer, is designed to receive an input of a small-sized received signal from the RFID tag 30, and its linearity is low. Accordingly, if a large transmission carrier leakage signal is inputted from the transmitting circuit 20, the LNA of the receiving circuit 25 is saturated, resulting in an inoperable state.
In order to solve this problem, Mitsubishi Electric Corp. of Japan has developed a canceller circuit 55 that removes the transmission carrier leakage signal, as illustrated in FIGS. 3A and 3B.
As illustrated in FIGS. 3A and 3B, the canceller circuit 55 is positioned on a line connecting the transmitting circuit and the receiving circuit with each other, and includes an amplitude phase adjustment circuit 75, an amplitude phase comparison circuit 73, a pair of integrators 78 and 79, an adder 77, and a pair of couplers 70 and 71.
The amplitude phase comparison circuit 73 detects the amplitude and the phase of the received signal inputted to the receiving circuit, and transfers an amplitude error and a phase error, which have passed through the integrators 78 and 79, respectively, to the amplitude phase adjustment circuit 75.
The amplitude phase adjustment circuit 75 receives a non-modulated signal generated from the transmitting circuit and the amplitude error and the phase error from the amplitude phase comparison circuit 73, and adjusts the amplitude and the phase of the received signals. In this case, the amplitude phase adjustment circuit 75 generates an adjusted signal by adjusting an unmodulated signal generated from the transmitting circuit so that the adjusted signal has an amplitude equal to that of the unmodulated signal and a phase opposite to that of the unmodulated signal.
The adder 77 adds the adjusted signal adjusted by the amplitude phase adjustment circuit 75 to the received signal to remove the diffracted transmission carrier leakage signal from the transmitting circuit.
However, since the related canceller circuit 55 is composed of a plurality of circuits, integrators 78 and 79, and others, and requires couplers 70 and 71 to receive the signals from the transmitting circuit and the receiving circuit, the circuit construction of the RFID reader becomes complicated. In addition, the errors may become greater due to the difference between the bidirectional output characteristics of the couplers 70 and 71, and the feedback circuit using the integrators 78 and 79 may become unstable due to its characteristics.